There has been known a method for producing a carbon fiber bundle including heat-treating a precursor fiber bundle in an oxygen atmosphere at 200 to 400° C. to convert it into a stabilized fiber bundle followed by carbonizing the stabilized fiber bundle in an inert atmosphere at 1000° C. or higher to obtain the carbon fiber bundle. The carbon fiber bundle obtained by this method is widely used industrially, particularly as a reinforcement fiber for composite materials, owing to excellent mechanical properties thereof.
However, fusion may occur between single fibers in the stabilization step of converting the precursor fiber bundle into a stabilized fiber bundle, wherein the fusion may cause process failure such as fluffing and bundle breakage in the stabilization step and the subsequent carbonization step (hereinafter, the stabilization step and the carbonization step may be integrated and denoted also as a heating step). It is known that selection of an oil agent to be adhered to the precursor fiber bundle is important to avoid this fusion, and a large number of oil agent compositions have been studied. For example, a silicone-based oil agent in which an amino-modified silicone, an epoxy-modified silicone, a polyether-modified silicone or the like is blended is frequently used as an oil agent composition because it has high heat resistance and effectively suppresses fusion (for example, Patent Document 1).
However, for such a silicone-based oil agent mainly composed of a silicone compound having the effect of preventing the fusion between single fibers, the silicone component undergoes a crosslinking reaction upon heating, resulting in an increase in viscosity. As a result, a viscous material derived therefrom may accumulate on the surfaces of fiber transporting rollers and guides in the production process of the precursor fiber bundle and in the stabilization step, and a fiber bundle may be wound around or got caught in these rollers and guides to result in thread breakage, thereby leading to reduction in operability. Moreover, the oil agent composition containing a silicone compound may have such a problem that it produces silicon compounds such as silicon oxide, silicon carbide and silicon nitride in the heating step, and the scale thereof reduces the stability of the heating step and the quality of a product.
For this reason, a non-silicone-based oil agent using a non-silicone component as the main component of an oil agent composition has been proposed for many years for improving the operability of the heating step. Examples of the non-silicone-based oil agent include a polybutene (refer to Patent Document 2), a blend of a polyoxyethylene higher aliphatic alkyl ether and an antioxidant (refer to Patent Document 3), a neopentyl alcohol derivative (refer to Patent Document 4), an alkyl or alkenyl thio fatty acid ester (refer to Patent Document 5), a polymeric amide compound (refer to Patent Document 6), an ammonium salt of a fatty acid ester (refer to Patent Document 7), a fluorochemical surfactant (refer to Patent Document 8), an aromatic composite ester and an amide compound (refer to Patent Document 9).
However, although a non-silicone-based oil agent has advantageous points such as no occurrence of silicon compounds in the heating step or use of an inexpensive raw material, it is often poorer in thermal stability than a silicone-based oil agent, which causes the occurrence of fluffing and bundle breakage due to the fusion in the heating step. In addition, since the mechanical properties of the produced carbon fiber bundle are also poorer than those in the case where a silicone-based oil agent is used, the opportunity of using a non-silicone-based oil agent as an oil agent composition for acrylic precursor fibers for carbon fibers was limited to a part of product classes.
On the other hand, there is proposed a technique for reducing silicon compounds derived from a silicone-based compound produced in the heating step by combining a silicone-based oil agent and a non-silicone-based oil agent (refer to Patent Documents 10 and 11). However, the technique has had a problem that the compatibility of a silicone compound with a non-silicone compound is low, and so it is impossible to uniformly adhere a mixture of the silicone compound and the non-silicone compound to the surface of the precursor fiber bundle. As a result, the effect of preventing the fusion between single fibers has been insufficient in the part in which the non-silicone compound is unevenly distributed, that is, in the part in which the silicone component is present in a small amount or is not substantially present, and it has been difficult to stably obtain a carbon fiber bundle excellent in mechanical properties.
Furthermore, there is proposed a technique for improving emulsification stability by adding an alkylene oxide-modified silicone to an oil agent containing a silicone and a non-silicone component (refer to Patent Documents 12 and 13). However, although an alkylene oxide-modified silicone has a certain effect to stabilize emulsification, the compatibilization effect of a silicone and a non-silicone component is not sufficient. As a result, adhesion of the oil agent component to the precursor fiber bundle is not uniform, and fusion between single fibers cannot be completely prevented. Therefore, it has been difficult to stably obtain a carbon fiber bundle excellent in mechanical properties.
As mentioned above, with respect to process stability and development of mechanical properties of a carbon fiber bundle, the use of only a non-silicone-based oil agent composition by related art tends to be poorer than the use of an oil agent composition using a silicone compound as the main component. Therefore, a high-quality carbon fiber bundle cannot be stably obtained. Further, when an oil agent composition having a reduced content of silicone compound is used, it is difficult to uniformly adhere the silicone compound and a non-silicone compound to the surface of the precursor fiber bundle. Therefore, a high-quality carbon fiber bundle cannot be stably obtained.
That is, the problem of the reduction in operability due to the production of silicon compounds in the heating step stemming from a silicone-based oil agent and the problem of the reduction in mechanical properties of the carbon fiber bundle due to a non-silicone-based oil agent are in an inextricably linked relation, and both of these problems have not been solved by related art.    Patent Document 1    JP11-12855A    Patent Document 2    JP58-5287B    Patent Document 3    JP60-43446B    Patent Document 4    JP04-33891B    Patent Document 5    JP61-15186B    Patent Document 6    JP3481342B    Patent Document 7    JP63-36365B    Patent Document 8    JP02-19232B    Patent Document 9    JP09-78340A    Patent Document 10    JP2000-199183A    Patent Document 11    JP2005-264384A    Patent Document 12    JP2003-55881A    Patent Document 13    JP2003-278084A